1. Field of the Invention
This invention relates generally to the field of battery capacity indicator circuits, and in particular to a battery capacity indicator circuit for use in battery powered devices having visual displays.
2. Description of the Prior Art
FIG. 1 is a graph which shows a plot of battery life versus battery terminal voltage for a battery operating at several discharge rates. As can be observed from the battery life curves, the battery life obtainable from the battery is a function of the load which is coupled to the battery. For a relatively high value resistive load, such as indicated by curve 10, a relatively long battery life is obtained. For a nominal resistive load, such as indicated by curve 15, a nominal value battery life is obtained. And for a relatively low value resistive load, such as indicated by curve 20, a relatively short battery life is obtained. It will be appreciated, that the actual battery life obtained is a function of both the load, which determines the actual discharge rate, and the type and size of battery in use, which determines the battery capacity. Primary batteries, such as carbon-zinc batteries, provide low energy capacities, whereas zinc -air and lithium batteries provide high energy capacities. Other primary batteries, such as silver-oxide, manganese, alkaline, and mercury, batteries fall in between these two extremes. Still other batteries, such as NiCd secondary batteries provide a much lower energy capacity than that of the primary batteries of equivalent size, however, this disadvantage is compensated for by the fact the batteries can be repeatedly charged, thereby providing very long battery life before the battery must be replaced. Because of this large variation in the battery energy capacities for the different types of batteries, most prior art battery capacity indicators, such as those utilized in battery powered communication devices, have resorted to simply measuring the terminal voltage either under load, or at a predetermined time interval after the load has been removed, which resulted in a go/no-go indication of the end of life (EOL) of the battery. Once the measured voltage fell below a predetermined threshold voltage, the prior art battery capacity indicators provided a sensible alert indicating the end of life of the battery, and that the battery should be replaced or recharged. As a result, many users of such battery operated devices often found themselves with dead batteries, as the prior art battery capacity indicators provided no indication of the rate at which the battery was being discharged, or an indication of the amount of energy remaining within the battery.